Any Mystic Megs out there with a large crystal ball, that could tell us what the annuallised inflationary rate for electricity price rises will be for the next 20 - 25 years?
The best guess we can make, is to assume that "the more things change, the more they stay the same" or to assume that "history may not repeat, but it does tend to rhyme".
Therefore, the best guess going forward is to assume something similar to the multi-decade trend which led up to the present time. Inflation around 4% and electricity price inflation about the same but with bigger ups and downs along the way.
But similar to the "uncertainty" over inflation and electricity prices, there's "uncertainty" over what interest rates will be.
Will interest rates remain close to zero for the next twenty years, or will the pick up again?
In past inflationary periods, interest rates have at times gone ballistic - perhaps they will do again, and perhaps the future will offer surprisingly high "return on investment" for people holding cash. In the 1980's and 1990's, interest rates were generally well above inflation, so there was a positive *real* return.
On balance, and for the sake of fairness to the customer, I think that when making a 20-year forward projection, it is best to use long-term historic trends (+4% per year).
But of course the temptation will always be for the figures to be nudged to be as high as possible.
The higher the rate of projected inflation, the greater the "profit" the customer can be shown.
The company with the wildest exaggeration of inflation expectations therefore quotes the highest return on investment, quickest payback or highest total payback, so slightly tilts the odds in their favour for winning the contract.
It's a difficult problem, but as far as I'm concerned: "a bird in the hand is worth two in the bush" - so a decent payback percentage in year one is far more important than dreaming of how rich I might get in twenty years time but which requires all the stars come into alignment as per the original assumptions.
Just a couple of percent "drift" in the actual v expected annual inflation can make a vast difference over twenty years.
7% annual return on £1000 makes a total of £3869 after twenty years.
10.5% annual return becomes £7366 after twenty years - almost twofold difference in total "payback" for only a 3.5% difference in assumptions.